Air Microbiology : Introduction, Microbial content, Method for measuring, Calculation and Acceptable Limit of Air Pollution
Introduction of Air Microbiology
Study of living microbes suspended in air is simple called Air Microbiology. Transmission of airborne microorganisms-They are sprayed by normally coughing and Sneezing. Air microorganisms are carried by dust particles and droplet nuclei. Physical environment stress like desiccation, humidity, temperature, radiation. Indoor air e.g. tubercle bacilli where as outdoor air are algae, protozoa, yeasts and molds. Mold spore are predominant, e.g Clasdosporium, Alternaria, Pullularia , Penicillium , Batrytis, Stemphylium. Bacterial species are spore forming and non spore forming. e.g. Micrococcus, Sarcina, etc.
Air borne diseases
Bacterial origin-
- Diphtheria
- Tubeculosis
- Meningitis
Air borne Viral Diseases
- Small Pox
- Measles
- Influelza
Air Borne Fungal Diseases
- Sytemic Mycosis
- Histoplasmosis
- Cryptococcosis
Airborne infection
Transmission of infection produced by respiratory droplets less than 5 micro meter
Droplet infection
Transmission of infection produced by respiratory droplets larger than 5 micro meter in size
Note: Adult man inhales about 15 m3 of air per 24 hours and baby about 1 m3 /24 hours
Microbial content of air
It is important to remember that spores and fragments of moulds are numerous than bacteria. Number of bacteria are present in air in any time depend on variety of factors. Location of air whether indoor or outdoor air. Number of people present. Amount of their body movement. Amount of disturbance of their clothing etc. Experiments showed that the proportion of dust particles and droplets nuclei reaching the lung depends on their size
- >5 mm retained in nose
- 1 mm reach lungs and retained in alveoli but
- 1 mm retained in the lung diminishes
- Infective or potentially infective droplets may also be liberate by various laboratory procedure.
List of bacteria commonly found in air
- Micrococcus
- Sarcina
- Staphylococcus aureus
- Streptococcus pyogenes
- Mycobacterium tuberculosis
- Pseudomonas aeruginosa
- Bacillus anthracis
- Bacillus subtilis
- Proteus vulgaris
Method for measuring bacterial content of air
Methods that measure bacteria carrying particles (chiefly larger particles ) settle down by gravity from air on to the exposed surfaces e.g. settle plate method. Methods that count the number of bacteria carrying particles in a given volume of air e.g. slit sampler and air centrifuge method.
Settle plate Method
Dry off any moisture in the culture media ( preferred media blood Agar), malt extract agar may use for molds. Open plates of culture media expose for specific time ( 30 minutes or 1 hour ).It is generally suitable to expose plates on tables and ledges about 1 meter above the ground. Incubate the plates at 37 °C for 24 hours. Count the colonies.
Slit sampler as shown above image
Air centrifuge
Air centrifuge in which sampled air passes along a tube(strip) lined with nutrient agar which rotates rapidly on its long axis. After sampling, strip removes from instrument and incubated for 48 hours . Count colonies.
Calculation
The level of bacterial contamination of air expresses usually as the number of bacteria-carrying particle per cubic meter or the bio load.
B= 1000N/RT bcpm-3
Where
B= bio load
BCP= bacteria carrying particle
N= no. of colonies
T= duration of time in minute
R= rate of air sampling in liter /minute
Ventilated room shows 150 m3 and 4000/m3 and settle plates values from 10 to 1000/m3 per minute. Most are contaminants. They are harmless saprophytes and commensal and only 0.01 to 0.1% are pathogenic.
Acceptable limit of air pollution
Upper limits of the bacterial count in air in various areas are as follows.
- 50 per cubic feet in home office and factories
- 10 per cubic feet in general operation theater
- 1 per cubic feet in operation theater for neurosurgery
References
- Jewetiz medical microbiology, 5th edition.
- Lippincott, William and Wilkin. USA.2001.
- Collin and Lynes, microbiological methods oxford, 1995
- Pelczar, Microbiology, McGraw-Hill inc, 1996
- https://envs.au.dk/en/research-areas/microorganisms-in-the environment/the-microbiology-of-the-air
- https://microbewiki.kenyon.edu/index.php/Aeromicrobiology
- https://microbiologysociety.org/publication/past-issues/microbes-in-the-air.html
- https://bmcpublichealth.biomedcentral.com/articles/10.1186/1471-2458-12-594
- https://www.hindawi.com/journals/ijmicro/2019/5320807/
- https://books.google.com.np/books